1. Field of the Invention
The invention relates to a radiant heater unit for heating a heater plate, particularly a glass ceramic hotplate with at least one cooking point, with a support for an electric radiant heater comprising at least one radiant heater resistor, such as a heater coil, which extends from an annular periphery of a heating field determining a fixed heating field size via a central zone into an inner zone of the heating filed.
2. Prior Art
It is desirable in numerous cooking processes to achieve a very short initial cooking time, i.e. at the start of the cooking process the product being cooked is carefully heated to a predetermined temperature level over a minimum period of time, after which with reduced power regulated by means of a temperature switch or controlled by means of a power control device final cooking takes place, without it being necessary to separately operate the electrical operating member for manually selecting a power level at the cooking point. This automatic control of the heat emission of the cooking point is desired in such a way that after passing once from the initial cooking phase into the final cooking phase, it does not automatically return to the initial cooking phase, unless it is completely switched off and by corresponding cooling is made ready again for carrying out the next initial cooking phase.
It is also desired in the case of radiant heater bodies that, after switching on a cooking point, radiant heat is supplied in the visible wave range in a minimum period of time, so that the cook is able to recognise by means of the visible glowing of the associated radiant heater that the cooking point is ready to operate and consequently as rapidly as possible a high radiant power density or heat output is available.
Attempts have already been made to obtain this thermal and optical behaviour of a radiant heater body in that on the periphery of the heating field a separate radiant heating resistor is provided, which is switched in during the initial cooking or heating phase and remains switched off after this phase. Such an externally, separately switchable heating resistor also exists for those cooking points which can be switched between two fixed heating field dimensions so that, as desired, it is possible to switch to cooking utensils having different plan view sizes. Admittedly such arrangements in part lead to good results, but the disconnection of the heating resistor switched in during the initial cooking phase during the final cooking phase leads to a relatively non-uniform, specific heating of the bottom of the cooking utensil. In addition, for this purpose power control devices with additional switches are required.
An object of the present invention is to provide a radiant heater body of the aforementioned type, in which the time from switching on up to reaching a visible glow and therefore also the initial cooking time can be considerably shortened compared with hitherto known radiant heater bodies, which in particular have at least one exposed heating resistor.
According to the invention this object is achieved with a radiant heater body of the aforementioned type in that an area of the radiant heater belonging to a partial power system forms the periphery of the heating field and as marginal heating over at least part of the initial cooking phase there is a radiant power density which is higher than an area of the radiant heater located within it and belonging to at least one further partial power system. The initial cooking marginal or edge heating is not switched off at the end of the initial cooking phase and instead reverts to within a small difference of its radiant power density compared with the radiant heater area located within it. A variable initial cooking circuit is obtained which, at during the initial cooking phase at least temporarily supplies a faster heat evolution in the marginal area of the radiant heater body, i. e. where as a rule due to the conventional bottom shape of cooking utensils there is the best contact between said bottom and the hotplate. The increased positive power difference during the initial cooking phase with which the marginal heating system is operated compared with the standard partial power system can also be used to ensure that the thus operated marginal heating system glows visibly in an extremely short time after switching on the cooking point and thereby optically indicates the full readiness of said cooking point to operate.
The described heat emission behavior of the initial cooking marginal heating system, which is also further operated in the final cooking phase, can e.g. be achieved in a simple manner in that the initial cooking marginal heating system is provided or switched substantially over the entire initial cooking phase with a relatively high power level.
A particularly easy switching from the initial cooking phase to the final cooking phase can e.g. be brought about in that a partial power system of the radiant heater is switched in substantially time-dependent manner, preferably by means of a temperature switch with high switching temperature difference or hysteresis. This temperature switch only disconnects at a relatively high temperature influencing or controlling its temperature sensor and only switches in or connects again at a relatively low temperature such as can be achieved through complete disconnection and corresponding cooling and which is not normally reached by the radiant heater body during final cooking. Instead of this or in addition thereto this behavior characteristic of the temperature switch can also be achieved in that thermal coupling of the temperature sensor of the temperature switch is made so low to the radiant heater or radiant heater body, that only on reaching the end of the initial cooking phase is the temperature sensor heated to the disconnection temperature by heat conduction and then due to low heat dissipation through corresponding insulation can no longer cool to its switch-on temperature during the final cooking. Thus, there is a time-dependant control of the initial cooking phase using switching members responding exclusively to temperature influences.
A particularly simple embodiment of the invention is provided in that during the initial cooking phase at least one predetermined part of the radiant heater, i.e. at least one heating resistor is switched off by short-circuiting. Thus, it is possible to achieve a significant increase in the power in the outer region of the heating field without particular effort and expenditure, said solution even being suitable for those very simple radiant heater bodies having only a single radiant heating resistor, i.e. only a single electric heating circuit.
In particular in place of such a short-circuit connection for at least one part of the radiant heater located within the periphery, it is possible to form at least one such inner part by means of a radiant heating resistor having a high positive temperature coefficient (PTC). The device influencing the transfer from the initial cooking phase into the final cooking phase can then be formed exclusively by the associated radiant heating resistor, because as a result of its characteristic behavior, the PTC resistor brings about the desired reversal.
An even further simplified and manufacturing favorable solution of the inventive problem can be provided in that the initial cooking marginal heating system is formed by a separate and in particular single-strand heating circuit extending over a maximum of only 360.degree., which is optionally bifilar and is i.e. returned backwards and forwards twice and which is preferably always connected parallel to the inner part of the radiant heater. As a result of its arrangement on the periphery of the heating field, said heating conductor can be much higher loaded than the heater coil located within it and e.g. taking up the remainder of the heating field, so that there is a much faster visible glowing of this area of the heating field, as well as a shorter initial cooking time.
In addition to the above-described measures, but also in place thereof, an advantageous solution of the objects of the invention can be achieved in that the initial cooking marginal heating system is connected to the support with a lower thermal conductivity coupling than the inner part of the radiant heater, so that the specific heat dissipation from the marginal heating system into the support is much lower than that of the inner part of the radiant heater and consequently the marginal heating system glows visibly much more rapidly after switching on. This lower specific heat dissipation can be achieved by different, relatively simple measures, e.g. by a lower specific surface contact between the associated heating resistor and the support, by using a support material with a lower specific thermal conductivity in the vicinity of the marginal heating system and by other similar measures. In this case, without using a separate control or regulating device, the initial cooking control device can be exclusively formed by the thermal conduction connection between the radiant heater and the support, because towards the end of the initial cooking phase only the characteristics of this thermal conduction connection are used.
A particularly low specific thermal conduction connection between the marginal heating system and the support can e.g. be achieved in that longitudinal portions of the associated heating resistor are arranged in substantially freely suspended and contact-free manner with respect to the support, i.e. can run in contact-free or taut manner between suspended parts in the manner of suspension bridge sections.
The longitudinal portions of the heating resistor with a lower thermal conduction coupling to the support can also be obtained in that they are located in regions of the support, which have a different thermal conductivity from the material thereof and which are e.g. formed by an insulating or thermally insulating material which, although unsuitable for the direct mounting of the heating resistor, still has very good insulating characteristics.
Such an insulating material is used for radiant heater bodies, e.g. as an underbedding of a cup or disk-shaped insulating support body, which although having lower thermal insulation values, is suitable for a reliable mounting of the heating resistor by direct embedding. In this case the relatively dimensionally stable insulating support body can be provided with openings in the vicinity of said longitudinal portions of the heating resistor and into which appropriately project upwardly directed projections of the embedding, in such a way that said projections substantially completely fill the openings, at least in plan view. At least partially in the height direction said projections are set back with respect to the front side belonging to the heating resistors and/or can be at least partly advanced with respect thereto. The insulating support body can e.g. be a relatively firm or solid molded article made from mineral fibers constituted by a material, such as that e.g. known under the trade name "Fiberfrax", whilst the underbedding is based on pyrogenic silicic acid.
If an interruption of an electric circuit is used for switching from the initial cooking phase to the final cooking phase, the initial cooking control device appropriately has a temperature sensor operating the associated switch and which is preferably thermally insulated by means of an insulating layer compared with the radiant heater, which can be simply achieved without special insulating measures in that the temperature sensor is embedded in the already present insulating material of the support, i.e. is located on the radiant heating resistor side remote from the hotplate. If the temperature sensor is constructed as an expansion rod sensor, it can be embedded in simple manner in the insulating support by mere insertion and its control or switching head can be located outside the support. This switch acting in the manner of a temperature protection switch can also be formed by a so-called Klixon thermostat, which cooperates with a heat conducting rod, which transfers the heat from the sensing point to the temperature sensor e.g. formed by a bimetallic sensor in the thermostat switching head.
These and further features of preferred further developments of the invention can be gathered from the description and drawings and individual features can be realised singly or in the form of subcombinations in an embodiment of the invention and in other fields.